The background arts in this technical field include Japanese Patent No. 2723182 Publication (Patent Literature 1). This publication describes “for capturing fine objects in a liquid medium by aligning them at half-wave length intervals at nodes of sound pressure in a standing wave sound field, a back electrode of an ultrasonic oscillator is made of multiple strip electrode pieces independently arranged parallel with each other, and such an electrode piece to which voltage is applied is electrically switched to an adjacent electrode piece to move a driving part of the ultrasonic oscillator and move the standing wave sound field, thereby moving the captured fine objects along the arrangement direction of the electrode pieces.” (see Abstract).
The arts also include Japanese Patent Laid-Open Publication No. 2004-24959 (Patent Literature 2). This publication describes “an ultrasonic noncontact filtering method and apparatus therefor where a ultrasonic oscillator and a reflection plate are arranged in parallel with each other in and along a flow path filled with a liquid medium, emitted ultrasonic waves are reflected by the reflection plate, and fine objects dispersed in the liquid medium are captured at nodes of sound pressure or antinodes of the sound pressure of a standing wave sound field generated in the flow path” (see Abstract).